Gibberellins (GAs) form a very large family of tetracyclic diterpenoid carboxylic acids that have the basic structure called ent-gibberellane. They regulate multiple processes in the life cycle of higher plants, which are essential for normal plant growth and development (Graebe, J. E. (1987) Annu. Rev. Plant Physiol., 38, 419–465; and Hooley, R. (1994) Plant Mol. Biol., 26, 1529–1555). Biologically active GAs, such as GA1, are produced from trans-geranylgeranyl diphosphate mediated by sequential reactions of cyclases in the plastids, membrane-associated monooxygenases at the endoplasmic reticulum, and soluble 2-oxoglutarate-dependent dioxygenases located within the cytoplasm (Hedden, P. and Kamiya, Y. (1997) Annu. Rev. Plant Physiol. Plant Mol. Biol., 48, 431–460; and Lange, T. (1998) Planta, 204, 409–419). The biosynthetic pathway of GA is well established.
Growth of rice plants (Oryza Sativa L.) is regulated by the endogenous level of biologically active gibberellin, GA1 (Kobayashi, M. et al., (1989) Plant Cell Physiol. 30(7): 963–969). It has been proposed that GA2-oxidase catalyzes the catabolism of biologically active GAs and their precursors in higher plants (Ross, J. J. et al., (1995) Plant J. 7: 513–523). GA2-oxidase genes have been cloned from thale cress (Arabidopsis thaliana), pea (Pisum sativum) and bean (Phaseolus coccineus) (Lester, D. R. et al., (1999) Plant J., 19:65–73; Martin, D. N. et al., (1999) Plant Physiol., 121:775–781; and Thomas S. G. et al., (1999) Proc. Natl. Acad. Sci. USA 96:4698–4703). Thomas et al. (supra) reported that the transcription levels of Arabidopsis GA2-oxidase genes, AtGA2ox1 and AtGA2ox2, were reduced in GA-deficient mutants, but the levels increased after treatment with GA3. Based on these results, these researchers concluded that GA2-oxidase is associated with the maintenance of the concentration of biologically active GAs in plant tissues.
The present inventors also reported the cloning and characterization of the rice GA2-oxidase gene, OsGA2ox1 (Sakamoto, T. et al. (2001) Plant Physiol. 125(3): 1508–16). The OsGA2ox1 gene product catalyzed the metabolism of GA20 into GA29, and GA1 into GA8. However, the transcription level of OsGA2ox1 was not affected by GA3-treatment.